Electroplating of chromium



'tion of the heated air in winter.

United States Patent ELECTROPLATING OF CHROMIUM Henry Brown, Huntington Woods, and Donald R. Millage,

St. Clair Shores, Mich, assignors to The Udyiite Research Corporation, Detroit, Mich., a corporation of Michigan No Drawing. Application April 22, 1953, Serial No. 350,500

9 Claims. (Cl. 204-=51) This invention relates to improvements in the electrodeposition of chromium from aqueous acidic hexavalent' hydrogen at the cathode and of oxygen and ozone at the insoluble anodes. The undesirable spray and mist of chromic acid results from the violent bursting of the multitudinous gas bubbles of high surface energy which are released by the electrolysis. The spray and mist resulting from this bubble bursting is of considerable volume because of the high current densities used in the plating, the low efficiency of chromium deposition and the use of insoluble anodes. Due to the corrosiveness and high toxicity to the workers of this spray and mist, and its well known deleterious contaminating effect on other plating baths such as nickel, copper, cadmium and zinc, it is necessary in large scale production to employ powerful ventilation to continuously remove the same as it is formed. For example, to carry away the mist formed in a 2,000 gallon tank installation requires an exhaust of as much as 10,000 cubic feet per minute to enable safe continuous production. The use of powerful ventilation is expensive not only from the standpoint of the initial installation cost and upkeep maintenance, but also because of the abstrac- Moreover, not all of the spray and mist is removed from the area of the plating tank even when powerful ventilation is employed and this is especially true on wide plating tanks. In the usual installation, a certain amount of chromic acid mist does contaminate the air, especially when stray air currents pass over the tank during electrodeposition. From the standpoint of toxicity, the maximum safe concentration of chromium trioxide is now considered to be 0.1 milligram per cubic meter of inhaled air. In addition to the disadvantages connected with adequate ventilation, there is the further disadvantage of the loss of chromic acid in the exhausted air which may amount to about 30% of the chromic acid which is used in the electrodeposition. The amount of chromic acid thus exhausted, together with that which is carried out on the surface of the articles themselves, actually represents more chromic acid than that which is used up in the formation of the chromium coating.

The problems arising as the result of the formation of chromic acid spray and mist in commercial chromium plating, were recognized almost from the very inception of chromium plating and various attempts have been made to prevent or to greatly minimize the formation of this spray and mist. As examples of attempted solutions, it has been suggested to form blankets on the surface of the bath. by the use of various oils, floating objects such as plastic pieces, and various wetting agents especially nonionic wetting agents. None of these attempts have been found to be eminently satisfactory, and all have left much to be desired. For example, non-ionic wetting agents are rapidly oxidized at the anode, and floating plastic pieces are too readily displaced when articles are placed in and taken out of the bath. Heretofore, no ordinary compound which is soluble in the bath has been found that will prevent or effectively minimize the formation of spray and mist and yet be stable to the extremely powerful oxidizing conditions existing at the insoluble anodes during the electrodeposition of chromium fromthe acidic hexavalent chromium baths.

Copending application, Serial No. 334,081, filed January 29, 1953, broadly discloses fluorocarbon sulfonic acids and salts as soluble additives for aqueous acidic solutions of hexavalent chromium which are capable of greatly minimizing the formation of spray and mist during electrolysis of such baths with insoluble or highly polarized anodes. However, the shorter chain or lower molecular weight fluorocarbon sulfonic acids of the said prior application which contain fluorocarbon sulfonic acids having 4 to 8 carbon atoms carrying fluorine atoms, while capable of completely preventing the formation of fine spray and mist in acidic hexavalent chromium plating baths operated at room temperatures or up to about F, are not as completely effective in preventing mist when the operating temperature is raised and in the range of about 100 F. to F. These shorter chain or lower molecular weight fluorocarbon sulfonic acids are less difficult to prepare than the longer chain members, and therefore their use even in the warmer chromium plating baths is commercially desirable. Therefore, it is the primary objective of the present invention to make possible the use of the shorter chain fluorocarbon sulfonic acids, such as perfluorohexyl sulfonic acid and perfluoro 4-methyl cyclohexane sulfonic acid, during electrolysis in the warn'ier chromium plating baths for the substantially complete suppression of fine spray and mist.

In accordance with the present invention is has been discovered that this objective may be accomplished by the use of such short chain fluorocarbon sulfonic compounds in conjunction with pyridine and methyl pyridines or picolines which may be alpha, beta or gamma methyl pyridines and mixtures thereof.

The fluorocarbon sulfonic compounds of this invention which are usable in aqueous acidic hexavalent chromium plating baths of conventional composition and which have been found to be improved in warm baths by use in conjunction with pyridine and methyl pyridines may be characterized by the formula hereinafter designated Formula I:

where RF represents a saturated fluorocarbon chain including straight, branched and cyclic fluorocarbon chains of 4 to 8 carbon atoms, and X is a cation. The cation X may represent hydrogen or may be a metallic ion from the groups including the alkali metals, alkaline earth metals, rare earth metals and heavy metals, such as NH4, Mg, Zn, Ca, Cr, Al, Ni, Cu, Ce, etc. These compounds can be considered to be a special type of anionic surfaceactive agent, and in the accomplishment of the objects of this invention the anionic radical constitutes the dominant surface-active portion. In comparison to the anionic radical, the cation is relatively unimportant even though additional quantities of certain cations have been observed to increase the stability of the foam, such as zinc and copper.

Typical representatives of the compounds covered by Formula I which are suitable as additives to hexavalent chromium baths and the optimum concentrations for use in such baths are given in Table I. It will be understood that the proportions set forth in Table I represent only optimum concentrations and that concentrations up to saturation may be used with good results.

The concentration of pyridine or methyl pyridine to be used with the shorter chain fiuorcarbon sulfonic compounds, exemplified by the compounds of Table I, may vary quite widely within the range of about 5 to 60 grams per liter with 25 to 30 grams per liter usually being adequate, especially with pyridine. The incorporation of such quantities of pyridine or methyl pyridines in a hexavalent chromium plating bath containing a fluorocarbon sulfonic acid of this invention has been found to com pletely eliminate the formation of spray and mist during electrolysis even when the temperature of the bath is as high as about 140 F. An even greater foam stability may be achieved by employing with pyridine or methyl pyridines and a fluorocarbon sulfonic compound a metallic ion such as zinc or copper. The use of zinc or other effective metallic ions makes possible a somewhat thicker and more stable foam. The bubbles comprising the foam appear to be stabilized against too rapid collapse through the mechanism of the formation of a colloidal zinc chromate in the cathode film during the liberation of hydrogen with the result that the entire foam is more stable. When zinc or copper or other effective metal ions are present in an amount of about 5 to grams/ liter, the proportion of pyridine or methyl pyridine which is required is reduced.

The optimum pyridine or picoline concentrations given do not lower by themselves the surface tension of the acidic hexavalent chromium baths more than about one or two dynes. However, in the presence of about 2 grams/liter of, for example, perfiuoro 4-methyl cyclohexane sulfonic acid, the surface tension is lowered about nine dynes/ cm. lower than that accomplished by the above fluorocarbon sulfonic acid alone.

When a picoline (alpha, beta or gamma, or mixtures) is' used in the acidic hexavalent chromium baths for the electrodeposition of chromium, there is a slow but gradual oxidation at the insoluble lead or lead alloy anodes of the material, leaving ammonium ions in the bath and increasing the trivalent chromium concentration of the bath. For example, the electrolysis of such chromium baths containing about grams/liter of picoline (mixed picolines) during regular chromium plating will leave about 7-15 grams/ liter of trivalent chromium in the bath. The presence of trivalent chromium in such concentra tions has some beneficial effects in the chromium plating bath in increasing the cathode efficiency somewhat and in apparently making a somewhat less stress deposit. It has been observed however that the high trivalent chromium concentration makes the bath more sensitive to the surface condition (cleanliness or passivity) of nickel and copper, and gray streaks or finger prints often show up in chromium plating Work that was cleaned by the same procedure that would yield a more uniform bright deposit from a similar chromium bath not containing the relatively high trivalent chromium (7-15 g./l.) content and also the limiting cathode current density is lowered somewhat. However, where a less stressed chromium deposit is desired, the presence of the trivalent chromium is beneficial. With pyridine, which is more resistant to anodic oxidation than the picolines, there is less build-up of trivalent chromium, and a fairly pure pyridine is preferred for general uses.

The additives of this invention illustrated in Table I may be made in accordance with the method set forth in U. S. Patent 2,519,983. To illustrate that process in greater detail, a suitable procedure for making perfluoromethyl cyclohexane sulfonic acid will be set forth. Paratoluene sulfonyl chloride is utilized as the starting material and is inserted in an electrolytic cell of the type described in Uv S. Patent 2,519,983 containing liquid hydrogen fluoride, and the mixture is electrolyzed for a period of hours to produce perfluoro 4-methyl cyclohexane sulfonyl fluoride. By hydrolyzing the cyclohexane sulfonyl fluoride, the corresponding perfluoro sulfonic acid or a salt of the latter is produced.

Although an ordinary non-fluorinated aliphatic sulfonic acid having a carbon chain containing at least 8 carbon atoms or an aromatic sulfonic acid having at least an 8 carbon alkyl chain attached to the benzene ring is fairly stable when merely dissolved in acidic hexavalent chromium plating baths, such materials are rapidly and completely oxidized when the plating solution is electrolyzed using insoluble anodes, such as lead, lead-tin, lead-antimony or other lead alloy anodes. With ordinary six to eight carbon chain surface-active sulfonic acids, for example, n-hexyl sulfonic acid, n-heptyl sulfonic acid, cyclo hexane sulfonic acid, methyl cyclohexane sulfonic acid, ethyl cyclohexane sulfonic acid, there is very little diminution of the fine spray and mist, and in order to substantially prevent spray and mist, ten carbon chain sulfonic acids must be used. In less than four hours of electrolysis, 1 gram per liter of these non-fluorinated sulfonic acids is oxidized away leaving inorganic sulfate as a decomposition product in the bath. The sulfate thus formed alters the usual ratio of chromic acid anhydride (CrOa) to sulfate and thus interferes with the plating range of the bath unless the excess sulfate is removed by precipitation. It will be apparent that if other catalyst radicals are being employed in the bath with or instead of sulfate, such for example as fluoride or fiuosilicate, that the formation of excess sulfate ion will correspondingly affect the chromic acid anhydride ratio to the catalyst radical with a comparable decrease in the plating range of the bath. Because of the increased difficulties of control of the bath caused by the breakdown in the bath of such non-fluorinated sulfonic acids, their use is undesirable. In contrast, the fluorocarbon sulfonic compounds of the type covered by Formula I when dissolved in the bath in conjunction with pyridine or methyl pyridines as above stated, completely resist oxidation during electrolysis even when extremely high potentials, temperatures or concentrations of chromic acid are employed. Fluorocarbon compounds other than sulfonic compounds have not been found to be suitable when used in conjunction with pyridine or methyl pyridines. For example, if a fluorocarbon carboxylic acid, such as, the compounds, perfluorocaproic acid CFa (CFz) 4COOH and perfluorocaprylic acid CF3(CF2)6COOH or their salts is added to chromic acid baths used for the electrodeposition of chromium, for example, baths containing the ratio of :1 of CrOa to S04, there results a darkish or much more dull chrominum deposit, that is, there is a marked decrease in brightness of the deposit which can be overcome only by the use of an unusual ratio of CrOs to S04, for example, 50:1 instead of the usual 100:1.

In a large proportion of commercial chromium plating, the chromium is applied over an underlayer of nickel. Where nickel is the underlayer, the fluorocarbon sulfonic compounds of this invention tend to render the underlying nickel plate less sensitive to passivation by contact with the chromic acid solution before current is applied. In forming a chromium layer over nickel, it has been found to be desirable in some instances to preliminarily cathodically gas (at low voltage) the nickel plated article as it is immersed in the hexavalent chromium plating bath and before the higher electroplating voltage is applied. Such mild cathodic gassing is accomplished by using voltages below the plating voltage for a short period of time. One suitable condition for such gassing is a potential of 2 to 3 volts maintained for about 5-30 seconds.

The examples given below set forth formulations of operative chromium plating baths useful for decorative or engineering purposes. It will be understood that other compounds covered by Formula I and exemplified by the compounds of Table I may be used in these typical formulations in the place of the particular examples given. Additionally, it is to be understood that mixtures'of the compounds of this invention may be employed'as well as the single compounds.

Example I 150-250 grams/liter chromic acid (C103) 1.5-3 grams/liter S04 ion 0.5-4 grams/ liter perfluoro 4-methyl cyclohexane sulfonic acid (potassium salt) 25 grams/liter pyridine Temperature, 20-40 C.

Cathode current density, 100-300 amps/sq. ft. (approximately -30 amps/sq. dm.)

In the formulation of Example I, catalysts other than the sulfate ion may be employed, such, for example, as the fluoride ion or the fluosilicate ion or mixtures thereof. The proportion of fluoride or fluosilicate ion employed is an amount which provides an equivalent catalytic effect to that which is provided by the 1.5-3 grams/liter of the sulfate ion.

Example 11 400 grams/liter chromic acid (CrOa) 3-4 grams/liter S04 ion 0.3-3 grams/liter perfluoro 2-, or 4-ethyl cyclohexane sulfonic acid 5-10 grams/ liter pyridine Temperature, 20 C.50 C.

Cathode current density, 100-500 amps/sq. ft.

In forming a chromium coating on underlayers of nickel or copper, it is desirable to maintain the ratio of CrOs to S04 between about 75 :1 and 150:1. Where the underlayer is white brass (20% Cu-80% Zn) it has been found that the ratio of Cr0 to S0 may be increased to as high as 200:1.

Example III 100 grams/ liter CIO3 120-220 grams/liter NazCrzOr or K2Cr20r S04 ion at 2-4 grams/liter or catalyst" equivalent in F,

SiFs ions or mixtures 0.2-4 grams/ liter perfluoro 2,4 dimethyl cyclohexane sulfonic acid 10 grams/ liter pyridine Temperature, 20 C.50 C.

Cathode current density, 150-300 amps/sq. ft.

Example IV 300-400 grams/liter CrOs 3 grams/liter S04 anion 0.5-2 grams/ liter perfluoro 4-methyl cyclohexane sulfonic acid 5-10 grams/liter zinc 5 grams/liter alpha or mixed beta-gamma picolines Temperature, 20 C.50 C.

Cathode current density, 150-300 amps/sq. ft.

6 Example V 300 grams/liter CrOs 2 grams/liter S04 0.2-2 grams/ liter perfluoro 4-methyl cyclohexane sulfonic acid 5 grams/liter picoline 20 grams/liter pyridine Temperature, 20 C.50 C.

Cathode current density, -300 amps/sq. ft.

An admixture of pyridine, picolines or mixtures thereof with the compounds of this invention represented by the formula RFSOsX may be used to replenish the bath and to maintain the optimum proportions of each in the bath as set forth above, although some variation in the quantities of each ingredient in the admixture will necessarily exist depending upon the particular compound selected for use. Because of the relatively low rate of depletion of the additives of this invention in the bath, the additives may be, if desired, admixed with the chromic acid and incorporated into the bath as the chromic acid is replenished.

What is claimed is:

1. In a process of electrodepositing chromium from aqueous acidic hexavalent chromium solutions, the improvement which consists in adding to the solution the combination of 5-60 grams/liter of a material selected from the group consisting of pyridine and picolines and mixtures thereof, and a small amount of a compound eflective to substantially reduce the formation of spray during chromium electrodeposition having the formula RFSOsX where RF represents a saturated fluorocarbon chain of 4 to 8 carbon atoms and X is a cation.

2. In a process of electrodepositing chromium from aqueous acidic hexavalent chromium solutions, the improvement which consists in adding to the solution the combination of 5-60 grams per liter of a material selected from the group consisting of pyridine and picolines and mixtures thereof, and at least about .05 grams/liter of a compound having the formula RnSOsX where RF represents a saturated per fluorocarbon chain of 4 to 8 carbon atoms and X is a cation.

3. In a process of electrodepositing chromium from aqueous acidic hexavalent chromium solutions, the improvement which consists in adding to the solution the combination of 5-60 grams/liter of a material selected from the group consisting of pyridine, picolines and mixtures thereof, 5-20 grams/liter of a metallic ion selected from the group consisting of zinc and copper, and at least about .05 grams/liter of a compound having the formula RFSOBX where Rn represents a saturated fluorocarbon chain of 4 to 8 carbon atoms and X is a cation.

4. A method of electrodepositing chromium over nickel which comprises the steps of incorporating in an aqueous acidic hexavalent chromium bath the combination of 5-60 grams/liter of a material selected from the group consisting of pyridine, picolines and mixtures thereof, 5-20 grams/liter of a metallic ion selected from the group consisting of zinc and copper, and at least about 0.05 grams/liter of a compound having the formula RFSO3X where Rn represents a saturated fluorocarbon chain of 4 to 8 carbon atoms and X is a cation, immersing a nickel coated article in the bath, cathodically gassing said article by applying a voltage to the bath less than that normally applied for plating, and thereafter electroplating chromium from said bath.

5. A bath for the electrodeposition of chromium comprising an aqueous acidic hexavalent chromium solution containing 5-60 grams/liter of a material selected from the group consisting of pyridine and picolines and mixtures thereof, and a small amount of a compound effective to substantially reduce formation of spray during chromium electrodeposition having the formula RFSOsX where Rn represents a saturated fluorocarbon chain of 4-8 carbon atoms and X is a cation.

6. A bath for the electrodeposition of chromium comprising an aqueous acidic hexavalent chromium solution containing 5-60 grams/liter of a material selected from the group consisting of pyridine and picolines and mixtures thereof, and at least about .05 grams/liter of a compound having the formula RFSO3X where RF represents a saturated per fluorocarbon chain of 4 to 8 carbon atoms and X is a cation.

7. A bath for the electrodeposition of chromium comprising an aqueous acidic hexavalent chromium solution containing 5-60 grams/liter of a material selected from the group consisting of pyridine, picolines and mixtures thereof, 5-10 grams/liter of a metallic ion selected from the group consisting of zinc and copper, and at least about 0.05 grams/ liter of a compound having the formula RFSOSX Where RF represents a saturated fluorocarbon chain of 4 to 8 carbon atoms and X is a cation.

8. In a bath for the electrodeposition of chromium comprising an aqueous acidic solution of hexavalent chromium, an additive consisting of a compound hav ing the formula RFSOSX Where RF represents a saturated fiuorocarbon chain of 4 to 8 carbon atoms and X is a cation in sutficient amount to substantially re- ".8 duce the formation of spray in said solution, in combination with a material selected from the group consisting of pyridine, picolines and mixtures thereof in an amount suflicient to enhance the effectiveness of said compound at elevated operating temperatures.

9. An additive for an aqueous acidic solution containing hexavalent chromium and said additive, said additive consisting of an admixture of a material selected from the group consisting of pyridine, picolines and mixtures thereof, and a compound having the formula RnSOaX, Where RF represents a saturated fluorocarbon chain of 4 to 8 carbon atoms, and X is a cation, said compound being present in sufficient amount to substantially reduce the formation of spray in said solution during chromium plating, and 'said material being present in an amount sufficient to enhance the effectiveness of said compound at elevated operating temperatures References Cited in the file of this patent .UNITED STATES PATENTS 2,195,409 Flett Apr. 2, 1940 

1. IN A PROCESS OF ELECTRODEPOSITION CHROMIUM FROM AQUEOUS ACIDIC HEXALVALENT CHROMIUM SOLUTION THE IMPROVEMENT WHICH CONSISTS IN ADDING TO THE SOLUTION THE COMBINATION OF 5-60 GRAMS/LITER OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF PYRIDINE AND PICOLINES AND MIXTURES THEREOF, AND A SMALL AMOUNT OF A COMPOUND EFFECTIVE TO SUBSTANTIALLY REDUCE THE FORMATION OF SPRAY DURING CHROMIUM ELECTRODEPOSITION HAVING THE FORMULA RFSO3X WHERE RF REPRESENTS A SATURATED FLUOROCARBON CHAIN OF 4 TO 8 CARBON ATOMS AND X IS A CATION. 